1. Field of the Invention
The present invention relates to a control device and a method for operating a fuel injector of an internal combustion engine.
2. Description of Related Art
Internal combustion engines operating according to the Otto or Diesel method and injecting fuel directly into the combustion chamber of the internal combustion engine are especially advantageous with regard to efficiency, emission behavior and power output. In order to utilize the advantages of this so-called direct injection to the fullest extent possible, highest demands are made on the metering accuracy of the injectors, especially when small injection quantities are involved, in particular in the case of jet-directed combustion methods.
The metering of minute fuel quantities is required especially during multiple injections, in particular for the startup, warm-up and heating of the catalytic converter of the internal combustion engine. Furthermore, the requirements on the metering precision are increased even further by the increasing injection pressures. From published German patent application document DE 10 2004 015745 A1, a method for operating an injector and for determining the time of flight of the valve needle of the injector is known, to which reference is made hereby.
The injectors known from the related art have a characteristic curve of the time of flight of the valve element of the injector as a function of the actuation period, which basically is able to be subdivided into three ranges. As a rule, there is a direct correlation between the time of flight and the injected fuel quantity: the longer the time of flight, the greater the injected fuel quantity under unchanged marginal conditions.
In a first range, the so-called partial travel range, the injector is actuated only very briefly, and a characteristics curve segment results that rises monotonously but not always linearly. In a second range, the so-called transition range, the time of flight drops again with an increasing actuation period of the fuel injector, so that a first point of inflection, or a local maximum, is attained between partial travel range and the transition range.
This transition range ends at a second point of inflection, or a local minimum. A third characteristics curve segment begins at an actuation period that is greater than actuation period T2 associated with the second point of inflection, in which third segment the characteristic curve of the time of flight rises monotonously again and has an extremely linear characteristic.
Since the position of the transition range and the times of flights of the valve element associated with the first and second points of inflection are individual for each injector and also vary across the service life of the injector, it is currently not possible to represent the partial travel range and the transition range of the characteristic curve for actuating the injector, in particular for metering minute injection quantities, with the required accuracy. This is the reason why currently only the so-called full travel range is triggered with regard to the characteristic curve, which makes it impossible to meter minute fuel quantities.